A surface of a window glass plate, a wall, or the like is usually contaminated by dusts or surface scratches thereon. A conventional method for evaluating the contamination is a visual inspection by an inspector. However, the evaluation by a visual inspection may not be quantified and has a miserable repeatability, because the evaluation results are dispersed among inspectors and may not be recorded.
As an evaluation method which replaces a visual evaluation, quantitative methods have been proposed such as a method for measuring optical characteristics (color difference, glossiness, luminance, haze factor, transmittance, etc) of an object to be evaluated, and a method for measuring the mass (weight) of deposits on a surface of an object.
The conventional quantitative methods as described above measure average data (i.e., surface average data for a measured area) of the optical characteristics of an object or the mass of deposits on an object. The average data do not include factors such as fine distribution and the like, so that it does not match to the visual sensation of human being for the aggregation of contamination and unevenness due to deposits and fine surface scratches.
For measuring the optical characteristics of an object to be evaluated, an image of a surface of an object is taken by means of an ordinary optical camera. In the case of taking an image of a contaminated surface of a glass plate made of a transparent material of a high reflectivity mounted in a building, it is difficult to take a clear image of the contaminated surface of the glass plate, because the images of a background and a camera itself reflected on the glass plate are taken by the camera. When an image of the surface of the glass plate is taken from a direction oblique thereto by a camera in order to prevent the reflected image of the camera itself from being taken, correct image information for the contaminated surface is not obtained.
In order to resolve these-problems, the following settings are required, i.e., (1) an object only is uniformly and brightly illuminated by a light source, and the light reflected on the object does not impinge upon a camera, and (2) a camera is positioned in front of an object, but the reflected light is prevented from impinging upon a camera in order not to take an image of the camera itself reflected on the surface of the glass plate.
Referring to FIG. 1, there is shown a conventional system for taking an image in a dark room in which an object, a ring light source and a camera are arranged. In a dark room 10, there are arranged an object 12, a ring fluorescent lamp 14 for illuminating the object 12, and a cover 19 for the fluorescent lamp to prevent the light from the lamp from directly impinging upon a lens 18 of a CCD camera 16.
According to this conventional system, it is possible to obtain a clear image of a surface of the object, because all of the components are arranged in the dark room so that taking the images of a background and the camera itself reflected on the surface of the object 12 is prevented, and the light reflected on the object 12 does not impinge upon the lens 18.
When an image of the contamination due to contaminants, scratches, color unevenness, and the like on a glass plate is intended to be taken using the conventional system, it requires much time to set dedicated jigs and devices.
The conventional system also has no portability and mobility, and may not take an image of an upright glass plate and a glass plate mounted in a building.